How do I remove an item from a Neutral seller?

[Quetra]

So I need to remove an item from a Goblin Merchant by trigger. I tried using Neutral Building - Remove item from Unit and Neutral Building - Remove item from all marketplaces and neither worked.

Then I read that you have to add the items by trigger first in order to remove them later, rather than adding them in the object editor. When I did that, no items were added either. What am I doing wrong? The Goblin Merchant is just a custom copy of the default Neutral one.

 

[Tasyen]

This action "add Unit/Item to marketplace" has a text note reading only affects Units with the ability

• "Sells items"

• "Sells Units"

Make sure your Unit has the requiered skill.

 

[Quetra]

I have changed that now but it doesn't achieve what I want exactly since the item now disappears once bought. Is there a way to add items to buildings through triggers with the Shop Purchase Item ability?